1. Field of the Invention
The present invention relates to a metal molding machine, and more particularly to a metal molding machine having a movable platen, which is movable between a fixed platen and an end frame while being guided by tie rods. The present invention further relates to a mold casting method, which is carried out using such a metal molding machine.
2. Description of the Related Art
Metal molding machines or injection molding machines have a fixed mold and a movable mold, which are held against each other, thereby jointly defining a mold cavity. A molten material is poured into the mold cavity in order to produce a casting. The movable mold needs to move relatively with respect to the fixed mold. According to Japanese Patent No. 2792431 and Japanese Utility Model Publication No. 05-038890, it has been proposed to guide a movable platen that supports a movable mold with a plurality of tie rods. The fixed mold is secured to a fixed platen, and the tie rods are disposed between the fixed platen and an end frame.
The movable mold is considerably heavy, which tends to cause the tie rods to flex unduly. If the tie rods are sufficiently thick and rigid, the tie rods are prevented from flexing under the weight of the movable mold. However, in this case, the molding machine is liable to be large in size and costly to manufacture.
According to Japanese Patent No. 2792431, the movable platen is supported by a linear guide mounted on a base. According to Japanese Utility Model Publication No. 05-038890, the movable platen is supported on a base by wheels.
Since the movable platen is supported by the linear guide or by wheels according to the above publications, the tie rods are prevented from flexing under the weight of the movable mold.
However, if the relative height of the linear guide disclosed in Japanese Patent No. 2792431 with respect to the movable platen that is held in contact with the linear guide is too small, then the tie rods are not prevented from flexing. Also, if the relative height of the linear guide with respect to the movable platen is too large, then the tie rods tend to flex in the opposite direction. Therefore, the linear guide is required to be highly accurate in terms of the height of the linear guide. Furthermore, the linear guide inevitably causes a certain amount of friction, which makes it difficult for the movable platen to move smoothly.
The wheels disclosed in Japanese Utility Model Publication No. 05-038890 also are required to be considerably accurate in terms of the height thereof. The wheels are mounted on the lower surface of the movable platen, and can appropriately bear the weight of the movable platen. However, since the heavy weight of the movable mold is not directly supported, a rotational moment is produced about a center of gravity of the movable mold, which tends to cause the tie rods to flex excessively.
The surfaces of the wheels are inevitably susceptible to wear in use. Since the surfaces of the wheels are gradually worn down, the wheels cannot maintain the desired height for prolonged periods of time, thus allowing the tie rods to flex excessively.